Most Windows 8.1 product keys can still be used to activate Windows 10, which remains supported.

"Full Version" often implies the OS is pre-activated with cracked licenses, which is illegal and can be disabled by Microsoft at any time.

Modified ISOs may have essential system components removed to save space, leading to frequent crashes. Official Way to Get Windows 8.1 (Legacy)

If your hardware is too old for Windows 11, Linux Mint is a free, fast, and secure alternative that looks very similar to the traditional Windows desktop.